A. Field of the Invention
The present invention relates to the coating of optical waveguide fibers with liquid-applied organic materials which are thereafter cured to form solid protective organic coatings on the fibers. More particularly, the invention relates to an improved method, and apparatus for carrying out the method, wherein the surface of the optical waveguide fiber is conditioned prior to the application of the liquid-applied organic coating material in order to improve the qualities of the glass/organic coating interface and thus the properties of the protective coating as cured.
B. Description of the Prior Art
The application of liquid-applied organic coatings to optical waveguide fibers for the purpose of protecting the fibers from damage is well known. Glass fibers including glass optical fibers are quite strong as first drawn from a heated glass preform or glass melt, exhibiting very high strength so long as the surfaces of the drawn fiber remain defect-free. However, the processing of glass optical fibers for the purpose of cabling and installation thereof for applications such as optical telecommunications requires handling of the fibers, and results in a substantial weakening thereof unless the fibers are protected from surface damage.
In order to preserve the inherent strength of glass optical fibers as drawn, protective coatings consisting most typically of thermoplastics or liquid applied curable plastic resins have conventionally been applied to the fibers immediately after they are drawn and prior to contact between the fiber and any other surface. With recent advances in manufacturing technology, optical fibers are being drawn at ever increasing speeds, and apparatus for applying protective plastic coatings thereto has therefore been under continual development to provide dependable coating performance at the higher draw speeds being employed. Problems attending the use of high draw speeds include the appearance of uncoated surface areas or skips on the fiber surface, the application of the coatings in a non-concentric fashion, and the application of coatings incorporating bubble defects or other inhomogeneities. Refinements in coating application systems currently under development have as their objective the avoidance of these various coating defects.
One successful approach to the problem of fiber coating at high coating rates has been that of pressure coating. This process involves applying the organic coating material as a viscous liquid to the surface of the optical fiber under high pressure conditions wherein the coating liquid is force-fed to the fiber surface.
Methods and apparatus for pressure-applying organic coating materials to an optical fiber being drawn through the apparatus are described in U.S. Pat. No. 4,264,649. In accordance with the method of that patent, concentricity of the fiber within the coating and avoidance of skips or bubbles in the coating layer are attained utilizing pressure feeding of the coating material into a specially designed reservoir incorporating a tapered fiber guide cone and a tapered sizing orifice, the guide cone and orifice together forming a conical channel for directing the coating material toward the fiber surface.
Particular attention has been directed to the avoidance of bubble inclusions in organic coatings applied to optical fibers at high drawing speeds. These bubbles are generally considered to arise due to the entrainment of air carried with the surface of the optical fiber into the coating reservoir, due to the high draw speeds employed. Published patent specification GB No. 2 105 618 A describes apparatus designed to reduce bubble incorporation in the coating by employing a partial vacuum in a chamber located above the surface of the liquid in the coating reservoir. The partial vacuum is generated by means of an air induction device which reduces pressure in the air atmosphere surrounding the optical fiber at the point where it enters the surface of the coating liquid in the reservoir.
A different approach to the problem of air entrainment in the coating layer involves stripping entrained air or bubbles from the surface of the fiber after the fiber has entered the coating liquid. Published patent specification GB No. 2 113 574 A describes an optical fiber coating reservoir comprising upper and lower chambers containing coating material and wherein the lower chamber is maintained at elevated pressure. This results in a flow of the coating material upwardly into the upper reservoir, forming a countercurrent which sweeps bubbles from the surface of the optical fiber prior to the exit thereof from the coating system.
Inherent in the normal operation of coating dies of the above-described types is a condition wherein a downwardly drawn meniscus is formed in the surface of the coating material at the point of entry of the optical fiber into the coating liquid. This phenomenon is more particularly described in GB No. 2 113 574 A noted above.
While the foregoing systems have been effective to reduce coating skips or bubble formation in the liquid applied coatings, further improvements in the quality of the fiber/coating interface as well as further reductions in the number of bubbles entrained in the coating layer are needed. For example, if the coating/fiber interface is not characterized by good bonding of the coating material to the glass surface of the fiber, delamination of the coating from the fiber can occur. Most delamination is thought to occur in the course of temperature cycling which can be experienced during cabling or after installation of the cables in a telecommunications system. Of course, the complete elimination of bubbles from the organic coating, which bubbles can cause optical loss from the optical fiber due to microbending effects, would also be desirable.
Therefore, it is a principal object of the present invention to provide a method for improving the characteristics of the interface between glass optical fibers and liquid-applied organic coatings thereon.
It is a further object of the invention to improve the uniformity and reduce the formation of entrained bubbles in organic coatings applied by the application of liquid organic coating materials to optical fibers during fiber drawing.
It is a further object of the invention to provide apparatus for the application of liquid-applied coating materials to optical fibers which improves the fiber-coating interface and the uniformity of the applied coating.
Other objects and advantages of the invention will become apparent from the following description thereof.